Fabrication of microstructured and nano-structured products known to be of interest in various industries include arrays of structured elements having optical applications, such as lenticular lenses, Fresnel lenses, light guides, diffusers, retro-reflective films, micro-lens arrays, brightness enhancement film (BEF) and LED arrays. Other applications include biomedical components, solar concentrators, micro-fluidic products, tissue culture media, micro-electrical-mechanical devices (MEMS), micro-acoustical devices, Chemical Mechanical Planerization (CMP), fuel cells, and other geometries that benefit from high speed, precision, microfabrication technology that provides high volume commercialization at economical cost.
The present invention has novel advantages because mold cost and fabrication time is reduced, which translates to faster scale-up and commercialization but also benefits from higher manufacturing speed than the prior art. The invention also allows the use of a wider range of materials than the prior art, including both thermoplastic and thermoset polymers, optionally loaded with other second phase or filler materials such as, for example, ceramic, glass or metal powders. Such latitude in prior art processing does not exist or creates significant tool wear. The present invention provides the ability to microform materials that can withstand higher use temperatures or that become polymer composites, having improved mechanical, electrical or optical properties which are of significant benefit for some end use applications, beyond the narrow range of typically used polymers.
The present invention adapts several commercially known techniques not previously used in microfabrication to achieve novel results.
In accordance with the present invention, polymeric products can be made by electrodepositing powdered polymer by means of a variation of the process generally known as powder coating. This process, sometimes referred to as solventless or dry painting, does not require the use of any liquids and therefore eliminates the problems associated with air entrapment. According to one aspect of the invention, powder is applied to the structured mold from the bottom up, eliminating the possibility of air being trapped and speed is only limited by the melt time and cure rate of the polymer.
The powder coating industry is well known for coating metal substrates. Recently, significant innovations have reduced both the cure temperature and cure time, thereby allowing temperature-sensitive substrates such as wood and PVC to be coated. Three of the major industry innovators are Rohm and Haas Powder Coatings, PPG Powder Coatings, and DuPont Powder Coatings. Some of the typical polymers used for the powder coating process are acrylics, generally recommended for extreme weather-resistance, epoxy resins for pipe and office furniture, epoxy-polyesters for light fixtures and shelving, polyesters for paneling, automotive components and garden furniture, and silicones for high-temperature applications such as barbecue grills.
Application equipment to dispense the powder is quite sophisticated. Complete systems from companies such as ITW-Gema and Wagner provide complete automated systems that apply powder electrostatically to coat parts on a conveyer-line and then cure the coated parts. Of specific interest is equipment which has been designed for continuous webs such as for coil coating. Powder is applied to moving steel coils at relatively high speed (20-30 ft min.) and thickness of 50-200 microns (0.002″-0.008″) then cured and wound up into rolls. This equipment is substantially similar to what would be required to make continuous rolls of microstructured film as described in this application.
Conventional powder coatings are heat cured at temperatures that range from 300° F. and higher. These are useful for fabrication processes that use metal molds or high melt temperature polymeric molds, but in some cases there are advantages to using polymeric molds that have lower temperature stability. For fabrication processes that use low temperature polymeric molds, low temperature powder coatings are of value. Of particular interest are some of the recently developed UV powder coatings, which can cure in 1-5 seconds at temperatures as low as 125-175° F. Low temperature curing powder coatings are also of value when combining different layers of polymers to achieve products that have specific physical, chemical or optical properties.
Powder particle sizes range from 5-20 microns in diameter but it is possible to achieve even smaller sizes. The ability to achieve small particle sizes is important to some aspects of this invention because in some applications, there is a need to replicate microstructures with high aspect-ratios or with very small functional features. In the case of a high aspect-ratio feature, such as a mold with a recessed microstructure only 10 microns wide and 50 microns deep (5:1 aspect ratio), the associated powder particles would have to be small enough to fill the recesses of the mold.